Corsair AF Receiver Mods

RX Audio Improvement

The Corsair II audio output chain suffers, among other things, from countless noisy RC4136 op-amps along with several undersized coupling capacitors. Unfortunately, there is no direct op-amp replacement due to the non-standard RC4136 pin-outs. However, here's an an easy audio mod that greatly improves the audio working with what we've got. Bear in mind that you will be sacrificing the notch and BP filters because we will be bypassing those circuits.

  1. Locate the IF/AF board on the underside of the Corsair
  2. We're going to tap the product detector audio output after one stage of buffering by U1D. For the highest possible audio quality you would use a higher quality op-amp (OPA2134, etc) but for now we'll use the existing RC4136
  3. Tap the audio at U1D-12: go directly rearward from U1D-12 and you'll see 10K and 100K resistors adjacent to U1. Tack solder a wire to the 100K resistor lead directly opposite U1D-12. Connect the other end of this wire to the AF Gain control (no need to disconnect existing wiring - just unplug connectors 7 & 8). Connect it to the left terminal (looking down on the bottom of the Corsair from the front.
  4. Feed the output (center terminal) of the AF Gain control to R83 after lifting the U4D end of R83 (heat with soldering iron from component side and lead will lift right out). I used RG-174 with center conductor connected to center terminal of AF Gain control and shield connected to right terminal of AF Gain control. Wire a short jumper wire from the right terminal of AF Gain control to the body of the adjacent notch control pot which is chassis grounded. For the R83 end of the RG174, attached the shield to U5-7 after sliding the heat sink to the left to expose the IC terminals.

What you now have is 10-20dB lower level of hiss and much richer cleaner audio.

Also, I highly recommend installing Inrad 2.8Khz filters in both i.f.'s (do the 9Mhz filter socket mod first) and aligning the ssb oscillators for 50-100hz extra lowend extension.

Speaker Connector

The very first time I removed the bottom cover from my Corsair II, I didn't realize there was a speaker mounted on the cover that was hard-wired to the chassis and subsequently broke off the wire and destroyed the speaker. You must always be careful - after unscrewing the cover screws, flip the cover backward (i.e., in back of the rear panel) to not break the wire. It was also obvious that the wire needed a detachable connector. I used Radio Shack p/n 2103292 but others have used crimpable butt connectors (often found in automobile wiring):